Constructor injection in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is constructor injection in NestJS?

Constructor injection is a way to provide dependencies to a class by declaring them as parameters in the class constructor. NestJS automatically creates and passes these dependencies when creating the class instance.

intermediate

How does NestJS know which dependencies to inject in the constructor?

NestJS uses TypeScript's metadata and decorators to identify the types of the constructor parameters. It then looks for providers registered for those types and injects them automatically.

beginner

Show a simple example of constructor injection in a NestJS service.

@Injectable()
export class CatsService {
  constructor(private readonly dogsService: DogsService) {}
}

Here, DogsService is injected into CatsService via the constructor.

intermediate

Why is constructor injection preferred over manual creation of dependencies?

Constructor injection helps keep code clean and testable by letting NestJS manage dependencies. It avoids tight coupling and makes swapping or mocking dependencies easier.

intermediate

What happens if a dependency is not registered as a provider but used in constructor injection?

NestJS will throw a runtime error saying it cannot resolve the dependency. All injected dependencies must be registered as providers in a module.

In NestJS, how do you declare a dependency to be injected via constructor injection?

A Call the dependency inside the constructor manually

B Import the dependency inside the constructor body

C Use a global variable for the dependency

D Add it as a parameter in the constructor with an access modifier like private or public

Constructor injection requires declaring dependencies as constructor parameters with access modifiers so NestJS can inject them automatically.

What decorator must a class have to be injectable in NestJS?

A @Module()

B @Controller()

C @Injectable()

D @Component()

The @Injectable() decorator marks a class as a provider that can be injected via constructor injection.

If a dependency is missing from the providers array in a module, what will happen during constructor injection?

A NestJS throws an error that it cannot resolve the dependency

B NestJS injects a default empty object

C NestJS ignores the dependency and continues

D NestJS logs a warning but injects null

All dependencies must be registered as providers; otherwise, NestJS throws an error at runtime.

Which of these is a benefit of constructor injection in NestJS?

A Makes code run faster by avoiding function calls

B Improves testability by allowing easy mocking of dependencies

C Automatically generates UI components

D Removes the need for modules

Constructor injection helps testability by allowing dependencies to be replaced with mocks during testing.

How does NestJS identify which instance to inject for a constructor parameter?

A By the type of the parameter using TypeScript metadata

B By the parameter name as a string

C By the order of parameters in the constructor

D By scanning the file system for matching files

NestJS uses TypeScript's type metadata to match constructor parameters to registered providers.

Explain constructor injection in NestJS and why it is useful.

Think about how NestJS creates class instances with needed services.

Describe what happens if you forget to register a dependency as a provider but try to inject it in a constructor.

Consider how NestJS finds dependencies to inject.